Possible Vapor Lock

[spiydr]

I know, this subject has been done to death. :)

I first suspected my issue might be vapor lock but after some testing I am not so sure.

It does seem to happen on warmer days and when the engine is good and warm, but it doesn't sputter and die, it instead shuts off like someone pulled the plug. When this happened the other day I pulled the gas line off the carb and positioned it above the carb and turned the key to see if I was getting gas. Low and behold it was pumping out plenty of gas. So I have to assume it is not vapor lock?

Up to this point I have replaced the coil, Ignition Control Module, spark plugs and wires. I have also jiggled just about every wire I can think when the problem is occuring.

The filter(silver can that the return line hooks to) is positioned correctly with the return line at 12 o'clock.

Any help diagnosing my issue will be greatly appreciated.

[fulsizjeep]

Are you running stock Motorcraft ignition? I have seen the distributor pickup wires get chaffed and short out causing quick shut down. I have seen power source for the electric choke get shorted to hot exhaust manifold and produce similar results also.

[spiydr]

Are you running stock Motorcraft ignition? I have seen the distributor pickup wires get chaffed and short out causing quick shut down. I have seen power source for the electric choke get shorted to hot exhaust manifold and produce similar results also.

Yes, stock ignition. I will go ahead and take a good look at all the wires.

I have the Jeep outside idling right now waiting for it to either shut off or heat up at which point I will shut it down then try and start it. I am hoping it wont start and I can test for spark and fuel again in the comfort of my own driveway.

[FSJ Guy]

Buy a spare ignition module. They can fail without warning.

When it does this again, take a (small) hammer and tap on the module. If you can start the engine again, the module is bad.

You can also replace the module with a GM 4 pin ignition module. You'll need a heatsink to do this properly. Google is your friend for how to wire it up.

[FSJ Guy]

Are you running stock Motorcraft ignition? I have seen the distributor pickup wires get chaffed and short out causing quick shut down. I have seen power source for the electric choke get shorted to hot exhaust manifold and produce similar results also.

When the choke power lead shorts out to the exhaust, it blows a fusible link and leaves you dead in the water. AMHIK. :rolleyes:

[spiydr]

Buy a spare ignition module. They can fail without warning.

When it does this again, take a (small) hammer and tap on the module. If you can start the engine again, the module is bad.

You can also replace the module with a GM 4 pin ignition module. You'll need a heatsink to do this properly. Google is your friend for how to wire it up.

Did that without even meaning to. Went to Autozone to get a spare ICM and when I went to leave the Jeep wouldnt start. I immediatly switched out the ICM and it still wouldnt start.

[seventynine]

So....it turns over but will not fire?

If so pull one of the plug wires and put a spare spark plug on it to ground and see if you get a spark to rule out and electrical issue.

Dean

[spiydr]

So....it turns over but will not fire?

If so pull one of the plug wires and put a spare spark plug on it to ground and see if you get a spark to rule out and electrical issue.

Dean

Thats the plan the next time it fails.

[spiydr]

Well wish me luck. I cant get the Jeep to stall while idling in the driveway, guess that was a long shot. Gonna take her out and drive until she dies. Got all the tools I need to hopefully diagnose the problem.

[FSJ Guy]

Did that without even meaning to. Went to Autozone to get a spare ICM and when I went to leave the Jeep wouldnt start. I immediatly switched out the ICM and it still wouldnt start.

I'm sorry, but that's funny, if only because I can see that happening to me, too!

As mentioned above, check for spark. You need air, spark and fuel. If you're missing one of them, you're dead in the water.

Someone in another thread mentioned the distributor pickup going bad. I believe you can buy JUST the pickup part from NAPA. It looks like a little magnet encased in a blue (usually) plastic housing with wires coming out of it going to a connector.

[skinnyD]

Well wish me luck. I cant get the Jeep to stall while idling in the driveway, guess that was a long shot. Gonna take her out and drive until she dies. Got all the tools I need to hopefully diagnose the problem.

Only us diehard FSJ'rs willingly go out driving hoping to die out on the road to diagnose a problem!:eek:

[joe]

I too vote ign module too. Common prob with the MC ign systems. They get hot and shut down. Then cool off and work again till next time it get hot. When you do your test run keep a plastic baggie in the car. If you're in town when it shuts down go into a stop-n-rob and get a cup of ice, put the ice in the baggie and set it on the module for a couple minutes. Had a F-150 with a dying module but had an ice chest with me on a 800 mile trip. ice in a plastic bread bag applied to the module allowed me to limp it home the last couple hundred miles.

[spiydr]

Of course she wouldnt die for me so I parked her in the driveway and went in to have some lunch. Came back out 20 min later and she wouldnt start.:thumbsup:

I immediately pulled the plug wire and hooked a lead up to it so I could see spark jump to the engine. No Spark. I tried to get spark a number of ways but it was a no go. So unless my technique was flawed I would say I found my problem. I did check the coil for voltage and it was good. So the issue is either the pickup in the dizzy or the ICM. Now since I had previously replaced the ICM when the problem was occuring I have to assume its the dizzy.

Now I am going to wait a little while and check for spark again since after a cool down period it always starts back up.

[spiydr]

I just went back out and had my little homemade spark detector still setup. I went ahead and tried to start it again and she fired right up and my spark detector was obviously showing spark.

So I definately have a intermittent spark issue that only occurs after the engine heats up. I think I am going to go ahead and purchase a new pickup for the dizzy and see if that solves my problem.

I'm starting to run out of things to replace. :D

[spiydr]

I too vote ign module too. Common prob with the MC ign systems. They get hot and shut down. Then cool off and work again till next time it get hot. When you do your test run keep a plastic baggie in the car. If you're in town when it shuts down go into a stop-n-rob and get a cup of ice, put the ice in the baggie and set it on the module for a couple minutes. Had a F-150 with a dying module but had an ice chest with me on a 800 mile trip. ice in a plastic bread bag applied to the module allowed me to limp it home the last couple hundred miles.

This has occured multiple times and the icm was cool to the touch, not to mention its new. Of course it could be possible that my new ICM is bad though.

[FSJ Guy]

I'm starting to run out of things to replace. :D

You won't be :D when you realize that your intermittent spark problem has you dropping in a new longblock with MSD ignition and a new exhaust system just because you've got everything apart. :p

[Gambler68]

Don't worry, vapor lock doesn't exist ;)

[spiydr]

Well I completed replacing the pickup and took her for a drive with no issue. I then parked it for 20 minutes and went to start it and it ACTUALLY started.

I'm sure the Jeep Gods will throw something else at me, they don't ever want you getting to comfortable.

[tazzerzz]

This is the exact same issue I was going through
starts cold fine, let it sit for about 20 minutes and no start
turned out to be corrosion within the female dist connector and the icm female connectors